This theodolite belonged to John Johnson (1771-1841), the surveyor general of Vermont. Its basic form--with the telescope mounted on the open side of a semi-circle--derives from the
design that Jonathan Sisson introduced in London in 1737. The horizontal circle and vertical arc are graduated every degree and read by vernier to 10 minutes. The vertical arc has a second scale marked "LINKS of CHAINS" that correlates angle of elevation with horizontal distances, and that is to be used when surveying sloping ground. The level vial on the compass face seems to be original with the instrument; the vial outside the compass may be a later addition. The signature refers to either the John Gilbert who worked in London in the years 1719-1750, or his son, also known as John Gilbert, who worked in London in the years 1751-1791.
Ref: Gloria Clifton, Directory of British Scientific Instrument Makers 1550-1851 (London,
1995), p. 112.
"John Johnson" in Abby M. Hemenway, ed., The Vermont Historical Gazeteer
(Burlington, Vt., 1868), vol. 1, pp. 596-599.
"John Johnson" in National Cyclopaedia of American History, vol. 17, pp. 290-291.
